I have created an open source hardware project for the purpose of converting Wii extension controllers to USB using the AVR atmega168 microcontroller. So far it only works for the nunchuck and classic controller and has serious limitations but I hope it helps some people. I have posted the source code and manual on google code: http://code.google.com/p/wii-ext-to-usb/

Here is the video I shot playing Street Fighter IV on the PS3. (nb: the turbo feature is not in the source as I wanted to make a general converter, not something just for fighting games ((I have made headway into programming macro functions for special SFIV moves though.. muhaha XD )) )